Tikz calendar with alternating coloring for date numbers

Question

My code is adapted primarily from Jake's answers verticle and weekday; as well as Berteun Damman's example.

Berteun Damman's line tiks@lib@cal@width=7 doesn't appear to make a difference so I have commented it out.

My goal is to make the date-numbers be red for odd-numbered days, and black for the rest.

Although this question addresses the same issue, I wasn't able to find the answer there. However, Stefan Kottwitz suggests using the if (odd) construction… but I haven't been able to make that work.

I have tried various variations of if (<date is odd>) [red] but have not been able to express <date is odd> correctly.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{calendar}
\makeatletter
\tikzstyle{week list}=[
    execute before day scope={
        \ifdate{day of month=1}{\ifdate{equals=\pgfcalendarbeginiso}{}
            {% On first of month, except when first date in calendar.
            \pgfmathsetlength{\pgf@x}{\tikz@lib@cal@month@xshift}
            \pgftransformxshift{\pgf@x}
            }
        }{}
}, 
    execute at begin day scope={%
        \pgfmathsetlength\pgf@y{\tikz@lib@cal@yshift}%
        \pgf@y=\pgfcalendarcurrentweekday\pgf@y%
        \pgftransformyshift{-\pgf@y}%
        },
                      execute after day scope={%
        \ifdate{Sunday}{%
        \pgfmathsetlength{\pgf@x}{\tikz@lib@cal@xshift}%
        \pgftransformxshift{\pgf@x}
        }{}%
    },
    % % This should be defined according to Berteun (I don't know why.)
    % tikz@lib@cal@width=7
    %   ]
    ]

    \tikzoption{day headings}{\tikzstyle{day heading}=[#1]}
    \tikzstyle{day heading}=[]
    \tikzstyle{day letter headings}=[
    execute before day scope={ \ifdate{day of month=1}{%
        \pgfmathsetlength\pgf@ya{\tikz@lib@cal@yshift}%
        \pgfmathsetlength\pgf@xa{\tikz@lib@cal@xshift}%
        \pgftransformxshift{\pgf@xa}
        \foreach \d/\l in {0/Monday,1/Tuesday,2/Wednesday,3/Thursday,4/Friday,5/Saturday,6/Sunday} {
        \pgf@ya=\d\pgf@ya%
        \pgftransformxshift{-\pgf@xa}%
        \pgftransformyshift{-\pgf@ya}%
        \node[every day,day heading, anchor=base]{\l};%
            } 
        }{}%
    }%
]
\makeatother

\begin{document}
\tikz
\calendar
    [name=cal,
        dates=2018-02-01 to 2018-04-last,
        week list,
        day letter headings,
        day heading/.style={red, xshift=-1em},
        month label above centered,
        month text = \%y- \%mt \%y-,
        day xshift = 5ex,
        day yshift = 5ex,
        month yshift = 13ex
        ];
\end{document}

Answer 1

Add \ifodd\pgfcalendarcurrentday\relax \color{red} \fi to execute at begin day scope. The day of the month is contained in \pgfcalendarcurrentday and to check whether a number is odd in tex you can use \ifodd.

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{calendar}
\makeatletter
\tikzstyle{week list}=[
    execute before day scope={
        \ifdate{day of month=1}{\ifdate{equals=\pgfcalendarbeginiso}{}
            {% On first of month, except when first date in calendar.
            \pgfmathsetlength{\pgf@x}{\tikz@lib@cal@month@xshift}
            \pgftransformxshift{\pgf@x}
            }
        }{}
},
    execute at begin day scope={%
        \ifodd\pgfcalendarcurrentday\relax \color{red} \fi %%% <-- added line
        \pgfmathsetlength\pgf@y{\tikz@lib@cal@yshift}%
        \pgf@y=\pgfcalendarcurrentweekday\pgf@y%
        \pgftransformyshift{-\pgf@y}%
        },
                      execute after day scope={%
        \ifdate{Sunday}{%
        \pgfmathsetlength{\pgf@x}{\tikz@lib@cal@xshift}%
        \pgftransformxshift{\pgf@x}
        }{}%
    },
    % % This should be defined according to Berteun (I don't know why.)
    % tikz@lib@cal@width=7
    %   ]
    ]

    \tikzoption{day headings}{\tikzstyle{day heading}=[#1]}
    \tikzstyle{day heading}=[]
    \tikzstyle{day letter headings}=[
    execute before day scope={ \ifdate{day of month=1}{%
        \pgfmathsetlength\pgf@ya{\tikz@lib@cal@yshift}%
        \pgfmathsetlength\pgf@xa{\tikz@lib@cal@xshift}%
        \pgftransformxshift{\pgf@xa}
        \foreach \d/\l in {0/Monday,1/Tuesday,2/Wednesday,3/Thursday,4/Friday,5/Saturday,6/Sunday} {
        \pgf@ya=\d\pgf@ya%
        \pgftransformxshift{-\pgf@xa}%
        \pgftransformyshift{-\pgf@ya}%
        \node[every day,day heading, anchor=base]{\l};%
            }
        }{}%
    }%
]
\makeatother

\begin{document}
\tikz
\calendar
    [name=cal,
        dates=2018-02-01 to 2018-04-last,
        week list,
        day letter headings,
        day heading/.style={red, xshift=-1em},
        month label above centered,
        month text = \%y- \%mt \%y-,
        day xshift = 5ex,
        day yshift = 5ex,
        month yshift = 13ex
        ];
\end{document}